The 21-year-old made his United debut towards the end of last season and has impressed in his 15 appearances in all competitions this term.
McTominay was picked in midfield ahead of club-record signing Paul Pogba for Wednesday's last-16 first leg in Spain, and Mourinho believes that his performance vindicated the decision.
"In the press conference, the questions were about Paul [Pogba] but the question should be about the kid (McTominay)," the Portuguese told reporters after the game.
"He was fantastic. He did everything well. He put pressure on Ever Banega and stopped him playing, he's their playmaker. I think Scott was brilliant.
"The midfield started really well and Paul made a big effort to go into the dynamic when he came on and he had a positive performance."
The return leg takes place at Old Trafford on March 13, with United winning 15 of their 18 games on home soil so far this season.